The SONET/SDH (Synchronous Optical Network/Synchronous Digital Hierarchy) has been standardized as a standard of the optical transmission technologies for implementing speeding-up of low-speed communication lines by hierarchically merging and multiplexing the low-speed communication lines.
The frame period in the SONET is set to be 125 μs. Further, the signal format and the like in the SONET are hierarchically defined as OC-n (Optical Carrier-level n). To date, the OC-n has been standardized up to OC-768.
On the other hand, the frame period in the SDH is set to be 125 μs similarly to the frame period in the SONET. Further, the signal format and the like in the SDH are hierarchically defined as STM-n (Synchronous Transport Module-level n). To date, the STM-n has been standardized up to STM-256.
Transmission speeds (i.e., bit rates) and the hierarchical structures of the SONET and the SDH are identical to each other where the level (n) of the OC-n is greater than or equal to 3 and the level (n) of the STM-n is greater than or equal to 1.
The relation between the frame period and the bit rate in the SONET/SDH will be explained with reference to FIG. 1. Specifically, FIG. 1A illustrates the relation between the frame period and the bit rate in OC-3. FIG. 1B illustrates the relation between the frame period and the bit rate in OC-12. FIG. 1C illustrates the relation between the frame period and the bit rate in OC-48. In FIG. 1, the vertical axis represents frame period whereas the horizontal axis represents bit rate.
As represented in FIGS. 1A through 1C, the frame size in the OC-n varies in accordance with values of n. Specifically, the OC-n frame size is expressed as “n×90×9” bytes. Further, the frame period in the SONET/SDH is constantly 125 μs regardless of values of n, as described above. Therefore, the bit rate of the OC-n is expressed as “n×51.84” Mbps.
On the other hand, the WDM-based OTN (Wavelength Division Multiplexing based Optical Transport Network) has been standardized as a platform for transparent transmission of a client signal. The term “client signal” herein refers to the OC-n and the STM-n in the SONET/SDH and the like. The frame structure of ODUk (Optical Channel Data Unit k) in OTUk (Optical Transport Unit k) defined for the OTN is equivalent to the frame structure of the OTUk excluding FEC (Forward Error Correction) and OH (Overhead).
The relation between the frame period and the bit rate in the ODUk will be hereinafter explained with reference to FIG. 2. In FIG. 2, the vertical axis represents frame period whereas the horizontal axis represents bit rate. FIG. 2 represents the relation between the frame period and the bit rate in ODU0, the ODU1 and the ODU2.
As represented in FIG. 2, the frame size of the ODUk is constantly “3824×4” bytes regardless of values of k. On the other hand, the frame period of the ODUk varies in accordance with values of k. For example, the frame period of the ODU0 is 98.35 μs. The frame period of the ODU1 is 48.97 μs. The frame period of the ODU2 is 12.19 μs. Therefore, the bit rate of the ODUk varies in accordance with values of k.
The bit rate of the ODU0 is 1244.16 Mbps. The bit rate of the ODU1 is 2498.78 Mbps (=“1244.16×2×239/238” Mbps). The bit rate of the ODU2 is 10037.27 Mbps (=“1244.16×8×239/237” Mbps). Although not represented in the figure, the bit rate of ODU3 is 40319.22 Mbps (=“1244.16×32×239/236” Mbps). Further, the bit rate of ODU4 is 104794.45 Mbps (=“1244.16×80×239/227” Mbps).
In association with the OTN, Japan Laid-open Patent Application Publication No. JP-A-2003-188919 discusses a cross-connect device configured to provide client signals with an exclusive line service through networks having different control systems (e.g., a plurality of carriers). The cross-connect device includes a mapping unit and a switching unit. The mapping unit is configured to map a client signal into the payload area of an OTN frame. The switching unit is configured to switch the frame that the client signal is mapped by the mapping unit in an ODUk sub-layer of an OTN layer.
As described above, the frame period of the OC-n in the SONET/SDH is constantly 125 μs regardless of values of n. Therefore, it is preferable to connect a plurality of cross-connect devices having a predetermined bit rate (of the OC-1, for instance) as a baseline for implementing a cross-connect processing of outputting a data signal to a different path in the OC-n with the TDM (Time Division Multiplexing) technique.
For example, a device herein assumed is a cross-connect device having the bit rate of the OC-1. In this case, a cross-connect processing is implemented for the OC-3 having a bit rate that is three times as fast as the bit rate of the OC-1 on a frame-by-frame basis through the parallel connection of three cross-connect devices intended for the OC-1.
In contrast, the frame periods of the ODUk in the OTN are not in an integral multiple relation. Specifically, the frame period of the ODU0 (i.e., 98.35 μs) is not obtained even if any one of the frame periods of the ODUk (k≧1) is multiplied with any one of integers, as described above. Unlike the case of the OC-n, it is difficult to implement a cross-connect processing in the ODUk with the TDM technique through the connection of a plurality of cross-connect devices having a baseline bit rate (of the ODU0, for instance). Consequently, different cross-connect devices are selectively used for different ODUk with different values of k in order to implement a cross-connect processing in the ODUk with the TDM technique.